from __future__ import print_function

from math import factorial


def lattice_paths(n):
    n = 2 * n  # middle entry of odd rows starting at row 3 is the solution for n = 1, 2, 3,...
    k = n / 2

    return factorial(n) / (factorial(k) * factorial(n - k))


if __name__ == '__main__':
    import sys

    if len(sys.argv) == 1:
        print(lattice_paths(20))
    else:
        try:
            n = int(sys.argv[1])
            print(lattice_paths(n))
        except ValueError:
            print('Invalid entry - please enter a number.')
